Semiconductor device and manufacturing method thereof

ABSTRACT

First, half etching is performed to a semiconductor layer formed on an insulating layer to form trenches at positions of slab-portion regions in which slab portions are to be formed. After filling the trenches with an insulating film, a resist mask which covers the semiconductor layer at a projecting-portion region in which a projecting portion is to be formed and whose pattern ends are located on upper surfaces of the insulating films is formed on upper surfaces of the semiconductor layer and the insulating film, and full etching is performed to the semiconductor layer with using the resist mask and the insulating film as an etching mask, thereby forming an optical waveguide constituted of the projecting portion and the slab portions. Thereafter, a first interlayer insulating film is formed to cover the optical waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent ApplicationNo. 2015-155195 filed on Aug. 5, 2015, the content of which is herebyincorporated by reference into this application.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a semiconductor device and amanufacturing method thereof, and can be effectively applied to, forexample, a semiconductor device incorporating a silicon photonics deviceprovided with a silicon optical waveguide with a rib structure.

BACKGROUND OF THE INVENTION

Japanese Patent Application Laid-Open Publication No. 2010-219456(Patent Document 1) discloses a pattern forming method including a stepof performing a first anisotropic etching using a first hard mask layeras an etching mask to a substrate, a step of forming a second hard masklayer on the substrate on which a step portion has been formed by thefirst anisotropic etching, and a step of performing a second anisotropicetching to the substrate with using the second hard mask layer as anetching mask.

SUMMARY OF THE INVENTION

When forming a silicon optical waveguide with a rib structure (alsoreferred to as ridge structure), a silicon layer is first etched halfwayin a thickness direction with using a first mask to form an upper stepportion of a convex shape, and the silicon layer is further etched inthe thickness direction with using a as convex shape. In the formingmethod described above, however, the silicon optical waveguide with therib structure becomes asymmetric in a cross section orthogonal to anoptical waveguide direction due to misalignment between the first maskand the second mask, and this causes the loss in opticalcharacteristics.

Other problems and novel features will be apparent from description ofthe present specification and the attached drawings.

In a semiconductor device according to an embodiment, an opticalwaveguide has a rib structure including a projecting portion which ismade of a semiconductor layer with a first thickness and slab portionswhich are formed integrally with the projecting portion and made of thesemiconductor layer disposed on both sides of the projecting portion andhaving a second thickness smaller than the first thickness. The opticalwaveguide is covered with a first interlayer insulating film, and aninsulating film different from the first interlayer insulating film isformed on each of upper surfaces of the slab portions and on an outerside of side surfaces of a protruding portion of the projecting portion.

In a manufacturing method of a semiconductor device according to anembodiment, an optical waveguide with a rib structure is formed by thefollowing process. First, a first resist mask having openings atpositions of slab-portion regions in which slab portions are to beformed is formed on an upper surface of a semiconductor layer of an SOIsubstrate, and the semiconductor layer is half-etched with using thefirst resist mask as an etching mask, thereby forming trenches in theslab-portion regions. After filling the trenches with an insulatingfilm, a second resist mask which covers the semiconductor layer of aprojecting-portion region in which the projecting portion is to beformed and whose pattern ends are located on upper surfaces of theinsulating films is formed on the upper surface of the semiconductorlayer and on the upper surfaces of the insulating films. Then, thesemiconductor layer is fully etched with using the second resist maskand the insulating films as an etching mask, thereby forming an opticalwaveguide including the projecting portion and the slab portions.

According to an embodiment, it is possible to provide a semiconductordevice incorporating a silicon photonics device provided with a siliconoptical waveguide with a highly symmetric rib structure.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an example of a configuration of asemiconductor device according to the first embodiment;

FIG. 2 is a plan view showing a main part of an optical device accordingto the first embodiment, in which main parts of a first optical signalline constituted of an optical waveguide whose cross section orthogonalto an optical waveguide direction has a quadrangular shape, a secondoptical signal line constituted of an optical waveguide whose crosssection orthogonal to the optical waveguide direction has a convexshape, a converting part of the first optical signal line and the secondoptical signal line and a grating coupler are illustrated;

FIG. 3A is a cross-sectional view showing a main part of the opticalwaveguide taken along the line A1-A1 of FIG. 2;

FIG. 3B is a cross-sectional view showing a main part of the opticalwaveguide taken along the line A2-A2 of FIG. 2;

FIG. 3C is a cross-sectional view showing a main part of the opticalwaveguide taken along the line A3-A3 of FIG. 2;

FIG. 3D is a cross-sectional view showing a main part of the opticalwaveguide taken along the line A1-A1 of FIG. 2;

FIG. 4 is a schematic diagram showing an optical modulator according tothe first embodiment;

FIG. 5 is a cross-sectional view showing a main part of the opticaldevice according to the first embodiment, in which main parts of thesecond optical signal line constituted of an optical waveguide whosecross section orthogonal to the optical waveguide direction has a convexshape, the grating coupler, a phase modulating part of the opticalmodulator and a germanium optical receiver are illustrated;

FIG. 6 is a cross-sectional view showing a main part of a manufacturingprocess of an optical device according to the first embodiment, in whichmain parts of the first optical signal line constituted of an opticalwaveguide whose cross section orthogonal to the optical waveguidedirection has a quadrangular shape, the second optical signal lineconstituted of an optical waveguide whose cross section orthogonal tothe optical waveguide direction has a convex shape, the grating couplerand the phase modulating part of the optical modulator are illustrated;

FIG. 7 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 6;

FIG. 8 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 7;

FIG. 9 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 8;

FIG. 10 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 9;

FIG. 11 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 10;

FIG. 12 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 11;

FIG. 13 is a cross-sectional view showing a main part of an opticaldevice according to the second embodiment, in which main parts of afirst optical signal line constituted of an optical waveguide whosecross section orthogonal to an optical waveguide direction has aquadrangular shape, a second optical signal line constituted of anoptical waveguide whose cross section orthogonal to the opticalwaveguide direction has a convex shape, a grating coupler, a phasemodulating part of an optical modulator and a germanium optical receiverare illustrated;

FIG. 14 is a cross-sectional view showing a main part of a manufacturingprocess of an optical device according to the second embodiment, inwhich main parts of the first optical signal line constituted of anoptical waveguide whose cross section orthogonal to the opticalwaveguide direction has a quadrangular shape, the second optical signalline constituted of an optical waveguide whose cross section orthogonalto the optical waveguide direction has a convex shape, the gratingcoupler and the phase modulating part of the optical modulator areillustrated;

FIG. 15 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 14;

FIG. 16 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 15;

FIG. 17 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 16;

FIG. 18 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 17;

FIG. 19 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 18;

FIG. 20 is a cross-sectional view showing a main part of an opticaldevice according to the third embodiment, in which main parts of a firstoptical signal line constituted of an optical waveguide whose crosssection orthogonal to an optical waveguide direction has a quadrangularshape, a second optical signal line constituted of an optical waveguidewhose cross section orthogonal to the optical waveguide direction has aconvex shape, a grating coupler, a phase modulating part of an opticalmodulator and a germanium optical receiver are illustrated;

FIG. 21 is a cross-sectional view showing a main part of a manufacturingprocess of an optical device according to the third embodiment, in whichmain parts of the first optical signal line constituted of an opticalwaveguide whose cross section orthogonal to the optical waveguidedirection has a quadrangular shape, the second optical signal lineconstituted of an optical waveguide whose cross section orthogonal tothe optical waveguide direction has a convex shape, the grating couplerand the phase modulating part of the optical modulator are illustrated;

FIG. 22 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 21;

FIG. 23 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 22;

FIG. 24 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 23;

FIG. 25 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 24; and

FIG. 26 is a cross-sectional view showing a main part of themanufacturing process of the optical device continued from FIG. 25.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

In the embodiments described below, the invention will be described in aplurality of sections or embodiments when required as a matter ofconvenience. However, these sections or embodiments are not irrelevantto each other unless otherwise stated, and the one relates to the entireor a part of the other as a modification example, details, or asupplementary explanation thereof.

Also, in the embodiments described below, when referring to the numberof elements (including number of pieces, values, amount, range, and thelike), the number of the elements is not limited to a specific numberunless otherwise stated or except the case where the number isapparently limited to a specific number in principle, and the numberlarger or smaller than the specified number is also applicable.

Further, in the embodiments described below, the components (includingelement steps) are not always indispensable unless otherwise stated orexcept the case where the components are apparently indispensable inprinciple.

Also, even when mentioning that a component is “made of A”, “made up ofA”, “has A” or “includes A” in the embodiments below, elements otherthan A are of course not excluded except the case where it isparticularly specified that A is the only element thereof. Similarly, inthe embodiments described below, when the shape of the components,positional relation thereof and the like are mentioned, thesubstantially approximate and similar shapes and the like are includedtherein unless otherwise stated or except the case where it isconceivable that they are apparently excluded in principle. The samegoes for the numerical values and ranges described above.

Further, in the drawings used in the following embodiments, hatching isused in some cases even in a plan view so as to make the drawings easyto see. Also, the components having the same function are denoted by thesame reference characters throughout the drawings for describing theembodiments, and the repetitive description thereof will be omitted.Hereinafter, typical embodiments will be described in detail based onthe drawings.

First Embodiment

In recent years, the development of the technology to realize an opticalcommunication module by fabricating a transmission line made of silicon(Si) and integrating various optical devices and electronic devices withusing an optical circuit constituted of the transmission line as aplatform, that is, the silicon photonics technology has been activelypursued.

The technology disclosed in the first embodiment is applied inparticular to an optical device among various devices constituting asemiconductor device using the silicon photonics technology. For thisreason, in the following description, a structure and a manufacturingmethod of an optical device integrated on an SOI (Silicon On Insulator)substrate will be described. In addition, in the following description,a transmission line for optical signal, a grating coupler and an opticalmodulator are mainly illustrated as examples of optical devices and amultilayer wiring with a two-layer structure is illustrated as anexample, but the present invention is not limited to these.

<<Configuration of Semiconductor Device>>

First, an example of a configuration of a semiconductor device accordingto the first embodiment will be simply described with reference toFIG. 1. FIG. 1 is a schematic diagram showing an example of theconfiguration of the semiconductor device according to the firstembodiment.

As shown in FIG. 1, data output from a silicon electronic circuit C1having a control circuit or a memory circuit formed therein istransmitted as an electric signal to an optical modulator P1 through asilicon electronic circuit (transceiver IC (Integrated Circuit)) C2. Theoptical modulator P1 is an optical device which converts the datatransmitted as an electric signal into an optical signal. For example,continuous wave laser light is incident on the optical modulator P1 froma light source LS. By operating the phase of the light in the opticalmodulator P1 to change the state of the optical signal, the datatransmitted as an electric signal can be made to correspond to the phasestate of light.

The optical signal modulated in the optical modulator P1 is output tothe outside from a semiconductor device SM through an optical coupler P2such as a grating coupler or a spot-size converter.

On the other hand, an optical signal input to the semiconductor deviceSM is transmitted to an optical receiver P4 through an optical couplerP3 such as a grating coupler or a spot-size converter. The opticalreceiver P4 is an optical device which converts the data transmitted asan optical signal into an electric signal. Then, the data converted intoan electric signal in the optical receiver P4 is transmitted to thesilicon electronic circuit C1 through a silicon electronic circuit(receiver IC (Integrated Circuit)) C3.

Electric wirings mainly made of a conductive material such as aluminum(Al), copper (Cu) or tungsten (W) (indicated by hatched arrows inFIG. 1) are used for the transmission of the electric signal from thesilicon electronic circuit C1 to the optical modulator P1 and thetransmission of the electric signal from the optical receiver P4 to thesilicon electronic circuit C1. Meanwhile, a transmission line foroptical signal (hereinafter, referred to as optical signal line) madeof, for example, silicon (Si) is used for the transmission of theoptical signal.

In addition, the silicon electronic circuit C1, the silicon electroniccircuit C2 and the silicon electronic circuit C3 are formed insemiconductor chips SC1, SC2 and SC3, respectively, and the opticalmodulator P1, the optical couplers P2 and P3 and the optical receiver P4are formed in one semiconductor chip SC4. These semiconductor chips SC1,SC2, SC3 and SC4 and the light source LS are mounted on one interposerIP to form one semiconductor device SM.

Note that the case in which the electronic device and the optical deviceare formed in different semiconductor chips has been described here, butthe present invention is not limited to this. For example, it ispossible to form the electronic device and the optical device in onesemiconductor chip.

<<Structure of Optical Device>>

Next, various structures of the optical waveguide will be described.There are various structures of the optical signal line, and an opticalsignal line whose cross section orthogonal to an optical waveguidedirection has a quadrangular shape and an optical signal line whosecross section orthogonal to the optical waveguide direction has a convexshape will be illustrated as examples in the first embodiment. Further,a grating coupler, an optical modulator and a germanium optical receiverwill be illustrated as examples.

<Optical Signal Line and Grating Coupler>

First, the optical signal line and the grating coupler will be describedwith reference to FIG. 2 and FIGS. 3A to 3D. FIG. 2 is a plan viewshowing a main part of optical signal lines and a grating coupleraccording to the first embodiment, in which a first optical signal lineOT1 constituted of an optical waveguide whose cross section orthogonalto an optical waveguide direction has a quadrangular shape, a secondoptical signal line OT2 constituted of an optical waveguide whose crosssection orthogonal to the optical waveguide direction has a convexshape, a converting part of the first optical signal line OT1 and thesecond optical signal line OT2 and a grating coupler GC, which areformed on the same substrate, are illustrated. FIG. 3A is across-sectional view showing a main part of the optical waveguide takenalong the line A1-A1 of FIG. 2, FIG. 3B is a cross-sectional viewshowing a main part of the optical waveguide taken along the line A2-A2of FIG. 2, and FIG. 3C is a cross-sectional view showing a main part ofthe optical waveguide taken along the line A3-A3 of FIG. 2. FIG. 3D is across-sectional view showing a main part of the optical waveguide takenalong the line A1-A1 of FIG. 2.

As shown in FIG. 2 and FIGS. 3A to 3C, the first optical signal lineOT1, the second optical signal line OT2 and the grating coupler GC areconstituted of a semiconductor layer (referred to also as SOI layer) SLmade of silicon (Si). This semiconductor layer SL is formed on a mainsurface of a semiconductor substrate SUB made of single crystal silicon(Si) with an insulating layer (referred to also as BOX layer or lowercladding layer) CL interposed therebetween. The thickness of theinsulating layer CL is, for example, about 2 to 3 μm and is relativelythickly formed, and thus the light propagating through the opticalwaveguide does not leak out to the semiconductor substrate SUB.

The cross section of the first optical waveguide OT1 orthogonal to theoptical waveguide direction has a quadrangular shape. A height (H1) ofthe first optical signal line OT1 is, for example, about 100 to 400 nm,and a height of 220 nm can be shown as a typical value. A width (L1) ofthe first optical signal OT1 in the cross section orthogonal to theoptical waveguide direction is, for example, about 100 to 500 nm, and awidth of 440 nm can be shown as a typical value. When a plurality of thefirst optical signal lines OT1 (two first optical signal lines OT1 areshown in FIG. 2 and FIGS. 3A to 3C) are formed in parallel, the shortestinterval (S1) between the first optical signal lines OT1 adjacent in thecross section orthogonal to the optical waveguide direction is, forexample, about 100 to 200 nm.

The semiconductor layer SL constituting the second optical signal lineOT2 has a rib structure. The cross section of the second optical signalline OT2 orthogonal to the optical waveguide direction has a convexshape, and the second optical signal line OT2 has a projecting portion.Namely, the second optical signal line OT2 is constituted of aprojecting portion (first plate portion) which is made of thesemiconductor layer SL with a first thickness extending in the opticalwaveguide direction and slab portions (second plate portions) which areformed integrally with the projecting portion and made of thesemiconductor layer SL disposed on both sides of the projecting portionand having a second thickness smaller than the first thickness.

A height of the projecting portion is the same as the height (H1) of thefirst optical signal line OT1, and a height (H2 p) of a protrudingportion of the projecting portion is, for example, about 50 to 200 nm. Awidth (L2 p) of the projecting portion in the cross section orthogonalto the optical waveguide direction is, for example, about 100 to 500 nm,and a width of 440 nm can be shown as a typical value. A width (L2 s) ofthe slab portion from an end of the second optical signal line OT2 to aboundary between the slab portion and the projecting portion in thecross section orthogonal to the optical waveguide direction continuouslychanges and is, for example, about 100 to 10000 nm, and a width of 500nm can be shown as a typical value. When a plurality of the secondoptical signal lines OT2 (two second optical signal lines OT2 are shownin FIG. 2 and FIGS. 3A to 3C) are formed in parallel, the shortestinterval (S2) between the projecting portions adjacent in the crosssection orthogonal to the optical waveguide direction is, for example,about 100 to 200 nm.

In the converting part of the first optical signal line OT1 and thesecond optical signal line OT2, a width (L3) of the first optical signalline OT1 and a width (L3 p) of the projecting portion of the secondoptical signal line OT2 are the same in the cross section orthogonal tothe optical waveguide direction. Meanwhile, a width (L3 s) of the secondoptical signal line OT2 gradually increases from the first opticalsignal line OT1 toward the second optical signal line OT2, and the width(L3 s) is, for example, about 0 to 10000 nm.

The semiconductor layer SL constituting the grating coupler GC has a ribstructure. The cross section of the grating coupler GC in the opticalwaveguide direction has a convex shape, and the grating coupler has aplurality of projecting portions separated from each other in theoptical waveguide direction. Namely, the grating coupler GC isconstituted of projecting portions (first plate portions) which are madeof a plurality of semiconductor layers SL separated from each other inthe optical waveguide direction and having the first thickness and slabportions (second plate portions) which are formed integrally with theprojecting portions and made of the semiconductor layers SL disposed onboth sides of the projecting portions (between adjacent projectingportions) and having the second thickness smaller than the firstthickness.

The grating coupler GC is an element which couples the laser lightincident from outside to the light propagating through the opticalwaveguide and emits the light propagating through the optical waveguideto the outside. The light propagating through the grating coupler GC isdiffracted and radiated in a predetermined direction by the periodicrefractive index modulation (formed by asperities on the surface)provided along the propagating direction on the optical waveguidesurface.

A height of the projecting portion of the grating coupler GC is the sameas the height (H1) of the first optical signal line OT1 in many cases.In addition, a height of the protruding portion of the projectingportion of the grating coupler GC is the same as the height (H2 p) ofthe protruding portion of the projecting portion of the second opticalsignal line OT2 in many cases.

Furthermore, as shown in FIG. 3D, the first optical signal line OT1, thesecond optical signal line OT2 and the grating coupler GC are coveredwith a first interlayer insulating film (referred to also as uppercladding layer) ID1, a second interlayer insulating film ID2 and aprotection film TC. The first interlayer insulating film ID1 and thesecond interlayer insulating film ID2 are made of, for example, siliconoxide (SiO₂). The protection film TC is made of, for example, siliconoxide (SiO₂), silicon oxynitride (SiON), PSG (Phospho Silicate Glass) orsilicon nitride (Si₃N₄). A first layer wiring M1 and a second layerwiring M2 described later are not formed above the first optical signalline OT1, the second optical signal line OT2 and the grating coupler GCin some cases. In addition, the protection film TC is not formed abovethe grating coupler GC which inputs and outputs an optical signal fromand to the outside in some cases.

The optical waveguide of the first embodiment is characterized in thatan insulating film IF1 different from the first interlayer insulatingfilm ID1 is formed on each of upper surfaces of the slab portions and onan outer side of side surfaces of the protruding portion of theprojecting portion in the second optical signal line OT2 and the gratingcoupler GC. The insulating film IF1 is made of, for example, siliconoxide (SiO₂) and an upper surface of the insulating film IF1 and anupper surface of the projecting portion are almost flush with eachother. The insulating film IF1 is not limited to silicon oxide (SiO₂).However, when a different material is used, another material having arefractive index different from that of the insulating layer CL and thefirst interlayer insulating film ID1 is to be formed, and this may causethe loss in optical characteristics.

<Optical Modulator>

Next, the optical modulator will be described with reference to FIG. 4and FIG. 5. FIG. 4 is a schematic diagram showing the optical modulatoraccording to the first embodiment. FIG. 5 is a cross-sectional viewshowing a main part of the optical device according to the firstembodiment, in which main parts of the second optical signal lineconstituted of an optical waveguide whose cross section orthogonal tothe optical waveguide direction has a convex shape, the grating coupler,a phase modulating part of the optical modulator and a germanium opticalreceiver are illustrated. The phase modulating part of the opticalmodulator shown in FIG. 5 corresponds to the cross section taken alongthe line B-B of FIG. 4.

As shown in FIG. 5, the optical modulator PC which changes an electricsignal into an optical signal is constituted of the semiconductor layerSL. Here, the optical modulator PC with a pin structure will bedescribed by way of example, but the present invention is not limited tothis.

The semiconductor layer SL constituting the optical modulator PC has arib structure. The cross section of the optical modulator PC orthogonalto the optical waveguide direction has a convex shape, and the opticalmodulator PC has a projecting portion. Namely, the optical modulator PCis constituted of, like the second optical signal line OT2, a projectingportion (first plate portion) which is made of the semiconductor layerSL with a first thickness extending in the optical waveguide directionand slab portions (second plate portions) which are formed integrallywith the projecting portion and made of the semiconductor layer SLdisposed on both sides of the projecting portion and having a secondthickness smaller than the first thickness. In addition, the projectingportion serves as a core layer OW through which light propagates. Thecore layer OW is made of intrinsic semiconductor, that is, i-type(intrinsic type) semiconductor.

In the phase modulating part PM, a p type semiconductor PR is formed byintroducing a p type impurity into the semiconductor layer SLconstituting the slab portion on one side (right side in FIG. 5) of thecore layer OW. This p type semiconductor PR is formed to be parallelwith the core layer OW. Further, an n type semiconductor NR is formed byintroducing an n type impurity into the semiconductor layer SLconstituting the slab portion on the other side (left side in FIG. 5) ofthe core layer OW. This n type semiconductor NR is formed to be parallelwith the core layer OW. Namely, the semiconductor layer SL between the ptype semiconductor PR and the n type semiconductor NR serves as the corelayer OW made of intrinsic semiconductor, and thus the pin structure isformed.

A height of the projecting portion constituting the core layer OW is thesame as that of the first optical signal line OT1 and the second opticalsignal line OT2 and a height of the protruding portion of the projectingportion constituting the core layer OW is the same as that of theprotruding portion of the projecting portion of the second opticalsignal line OT2.

The light (for example, continuous wave laser) incident from an inputpart is divided to two optical waveguides at a branching part and phasesthereof are operated in respective phase modulating parts PM. In thephase modulating part PM, the carrier density in the core layer OW madeof intrinsic semiconductor changes by applying a voltage to each of thep type semiconductor PR and the n type semiconductor NR, and therefractive index in this region changes. Consequently, the actualrefractive index to the light propagating through the optical modulatorPC changes, so that the phase of the light output from the opticalmodulator PC can be changed.

The optical modulator PC is covered with the first interlayer insulatingfilm ID1. The first interlayer insulating film ID1 is made of, forexample, silicon oxide (SiO₂) and the thickness thereof is, for example,about 2 to 3 μm.

The first layer wiring M1 is formed on the first interlayer insulatingfilm ID1. The first layer wiring M1 is constituted of a main conductivematerial made of, for example, aluminum (Al), copper (Cu) oraluminum-copper (Al—Cu) alloy and barrier metal formed on a lowersurface and an upper surface of the main conductive material. Thebarrier metal is provided for preventing the diffusion of the metal ofthe main conductive material constituting the first layer wiring M1 andis made of, for example, tantalum (Ta), titanium (Ti), tantalum nitride(TaN) or titanium nitride (TiN). The thickness thereof is, for example,about 5 to 20 nm.

The optical waveguide of the first embodiment is characterized in thatthe insulating film IF1 different from the first interlayer insulatingfilm ID1 is formed on each of upper surfaces of the slab portions and onan outer side of side surfaces of the protruding portion of theprojecting portion in the optical modulator PC. The insulating film IF1is made of, for example, silicon oxide (SiO₂) and an upper surface ofthe insulating film IF1 and an upper surface of the projecting portionare almost flush with each other. The insulating film IF1 is not limitedto silicon oxide (SiO₂). However, when a different material is used,another material having a refractive index different from that of theinsulating layer CL and the first interlayer insulating film ID1 is tobe formed, and this may cause the loss in optical characteristics.

Further, first connection holes (referred to also as contact holes) CT1reaching the p type semiconductor PR and the n type semiconductor NR areformed in the first interlayer insulating film ID1 and the insulatingfilm IF1. In the first connection hole CT1, a first plug (referred toalso as buried electrode or buried contact) PL1 made of tungsten (W) asa main conductive material is formed together with barrier metal. Thebarrier metal is provided for preventing the diffusion of the metal ofthe main conductive material constituting the first plug PL1 and is madeof, for example, titanium (Ti) or titanium nitride (TiN). The thicknessthereof is, for example, about 5 to 20 nm. Through the first plug PL1,the p type semiconductor PR is electrically connected to the first layerwiring M1 and the n type semiconductor NR is electrically connected tothe first layer wiring M1.

The first layer wiring M1 is covered with the second interlayerinsulting film ID2. The second interlayer insulating film ID2 is madeof, for example, silicon oxide (SiO₂) and the thickness thereof is, forexample, 1 μm or more.

The second layer wiring M2 is formed on the second interlayer insulatingfilm ID2. The second layer wiring M2 is constituted of, like the firstlayer wiring M1 described above, a main conductive material made of, forexample, aluminum (Al), copper (Cu) or aluminum-copper (Al—Cu) alloy andbarrier metal formed on a lower surface and an upper surface of the mainconductive material. The barrier metal is provided for preventing thediffusion of the metal of the main conductive material constituting thesecond layer wiring M2 and is made of, for example, tantalum (Ta),titanium (Ti), tantalum nitride (TaN) or titanium nitride (TiN). Thethickness thereof is, for example, about 5 to 20 nm.

Second connection holes (referred to also as via holes) CT2 reaching thefirst layer wirings M1 are formed in the second interlayer insulatingfilm ID2. In the second connection hole CT2, a second plug (referred toalso as buried electrode or buried contact) PL2 made of tungsten (W) asa main conductive material is formed together with barrier metal. Likethe barrier metal of the first plug PL1 described above, the barriermetal is provided for preventing the diffusion of the metal of the mainconductive material constituting the second plug PL2 and is made of, forexample, titanium (Ti) or titanium nitride (TiN). The thickness thereofis, for example, about 5 to 20 nm. The first layer wiring M1 iselectrically connected to the second layer wiring M2 through the secondplug PL2.

The second layer wiring M2 is covered with the protection film TC and anupper surface of the second layer wiring M2 is exposed by forming anopening in a part of the protection film TC.

<Germanium Optical Receiver>

Next, the germanium optical receiver will be described with reference toFIG. 5. Since germanium (Ge) and silicon (Si) have high affinity foreach other, the germanium optical receiver can be monolithically formedon the semiconductor layer SL made of silicon (Si).

As shown in FIG. 5, for example, the germanium optical receiver PD has avertical-type pin structure, and is constituted of a p type layer PSobtained by introducing a p type impurity into the semiconductor layerSL, a germanium layer GE formed on the p type layer PS and an n typelayer NS formed on the germanium layer GE. The n type layer NS is madeof, for example, silicon germanium (SiGe) and an n type impurity isintroduced thereto.

The p type layer PS is covered with the first interlayer insulating filmID1 and is electrically connected to the first layer wiring M1 throughthe first plug PL1 embedded in the first connection hole CT1 formed inthe first interlayer insulating film ID1. Similarly, the n type layer NSis covered with the first interlayer insulating film ID1 and iselectrically connected to the first layer wiring M1 through the firstplug PL1 embedded in the first connection hole CT1 formed in the firstinterlayer insulating film ID1.

<<Manufacturing Method of Optical Device>>

A manufacturing method of an optical device according to the firstembodiment will be described in order of process with reference to FIG.6 to FIG. 12. FIG. 6 to FIG. 12 are cross-sectional views each showing amain part of an optical device in the manufacturing process according tothe first embodiment. A region A indicates the first optical signal line(optical waveguide whose cross section orthogonal to the opticalwaveguide direction has a quadrangular shape), a region B indicates thesecond optical signal line (optical waveguide whose cross sectionorthogonal to the optical waveguide direction has a convex shape), aregion C indicates the phase modulating part of the optical modulatorand a region D indicates the grating coupler. The cross sectionorthogonal to the optical waveguide direction is shown for each of theregion A, the region B and the region C, and the cross section in theoptical waveguide direction is shown for the region D.

In the manufacturing method of a semiconductor device according to thefirst embodiment, the first optical signal line OT1, the second opticalsignal line OT2, the optical modulator PC and the grating coupler GC areformed. The full etching and the half etching are used for theprocessing of the semiconductor layer SL. The full etching means thatthe semiconductor layer SL is dry-etched from the upper surface to thelower surface through the whole thickness thereof, and the half etchingmeans that the semiconductor layer SL is dry-etched from the uppersurface while leaving a predetermined thickness thereof.

First, as shown in FIG. 6, an SOI substrate (substrate with anapproximately circular planar shape referred to as SOI wafer in thisstage) including the semiconductor substrate SUB, the insulating layerCL formed on the main surface of the semiconductor substrate SUB and thesemiconductor layer SL formed on the insulating layer CL is prepared.

The semiconductor substrate SUB is a support substrate made of singlecrystal silicon (Si), the insulating layer CL is made of silicon oxide(SiO₂) and the semiconductor layer SL is made of silicon (Si). Thethickness of the semiconductor substrate SUB is, for example, about 750μm. The thickness of the insulating layer CL is, for example, about 2 to3 μm. The thickness of the semiconductor layer SL is, for example, about100 to 400 nm and it is set to 220 nm by way of example in this case.

Next, a first resist mask RM1 for processing the semiconductor layer SLis formed. The first resist mask RM1 is formed by, for example, applyingphotoresist on the semiconductor layer SL and then patterning thephotoresist by performing the immersion exposure using the ArF excimerlaser (wavelength: 193 nm) and the development thereto.

In this case, the first resist mask RM1 is formed so as to expose thesemiconductor layer SL to be the slab portion of the optical waveguideof the second optical signal line OT2 in the region B, expose thesemiconductor layer SL to be the slab portion of the optical waveguideof the optical modulator PC in the region C and expose the semiconductorlayer SL to be the slab portion of the optical waveguide of the gratingcoupler GC in the region D. Namely, the first resist mask RM1, whichdetermines the width of the projecting portion and the width of the slabportion in the direction orthogonal to the optical waveguide directionin the region B and the region C and determines the width of theprojecting portion and the interval between the adjacent projectingportions (width of the slab portion) in the optical waveguide directionin the region D, is formed. Note that, when the processing depth by thehalf etching is different in the regions B and C and the region D,processes may be separately performed for the respective depths.

Next, as shown in FIG. 7, the semiconductor layer SL is processed byhalf etching with using the first resist mask RM1 as an etching mask.Thus, a plurality of trenches are formed in the semiconductor layer SL.The depth of the trench is, for example, about 70 nm. The trenchdetermines the width of the projecting portion and the width of the slabportion in the direction orthogonal to the optical waveguide directionin the region B and the region C and determines the width of theprojecting portion and the interval between the adjacent projectingportions (width of the slab portion) in the optical waveguide directionin the region D. In addition, the thickness of the semiconductor layerSL left under the trench corresponds to the thickness of the slabportion of each optical waveguide.

Further, the semiconductor layer SL to be the projecting portion of theoptical waveguide of the second optical signal line OT2 is formed in theregion B, the semiconductor layer SL to be the projecting portion of theoptical waveguide of the optical modulator PC is formed in the region C,and the semiconductor layer SL to be the projecting portion of theoptical waveguide of the grating coupler GC is formed in the region D.

As the etching gas for the half etching, for example, chlorine-based(Cl₂-based) gas, hydrogen bromide-based (HBr-based) gas, carbontetrafluoride-based (CF₄-based) gas or sulfur hexafluoride-based(SF₆-based) gas is used. After the half etching, the first resist maskRM1 is removed by oxygen (O₂) plasma ashing and then the RCA clean isperformed. Thereafter, a natural oxide film and others formed on thesurface of the semiconductor layer SL are removed by the wet etching.

Next, as shown in FIG. 8, the insulating film IF1 is formed on thesemiconductor layer SL by, for example, the SA-CVD (Sub-AtmosphericChemical Vapor Deposition) so as to fill each of the plurality oftrenches formed in the semiconductor layer SL. The insulating film IF1determines the width of the projecting portion and the width of the slabportion in the direction orthogonal to the optical waveguide directionin the region B and the region C and determines the width of theprojecting portion and the interval between the adjacent projectingportions (width of the slab portion) in the optical waveguide directionin the region D.

The insulating film IF1 is made of, for example, silicon oxide (SiO₂)and the thickness thereof is, for example, about 140 nm. Note that theinsulating film IF1 is not limited to silicon oxide (SiO₂), and anyinsulating film can be used as long as it can have etching selectivitywith respect to silicon (Si) constituting the semiconductor layer SL. Asdescribed above, however, if the refractive index is taken intoconsideration, silicon oxide (SiO₂) is preferable.

Next, the upper surface of the insulating film IF1 is ground by, forexample, the CMP (Chemical Mechanical Polishing), thereby filing each ofthe plurality of trenches formed in the semiconductor layer SL with theinsulating film IF1.

Next, as shown in FIG. 9, a second resist mask RM2 for processing thesemiconductor layer SL is formed. The second resist mask RM2 is formedby, for example, applying photoresist on the semiconductor layer SL andthen patterning the photoresist by performing the immersion exposureusing the ArF excimer laser (wavelength: 193 nm) and the developmentthereto.

Here, the second resist mask RM2 is formed in the region A so as tocover the semiconductor layer SL to be the optical waveguide of thefirst optical signal line OT1. Also, the second resist mask RM2 isformed in consideration of the alignment error in the region B so thatthe semiconductor layer SL to be the optical waveguide of the secondoptical signal line OT2 is covered and pattern ends of the second resistmask RM2 are certainly located on the insulating films IF1. In addition,the second resist mask RM2 is formed in consideration of the alignmenterror in the region C so that the semiconductor layer SL to be theoptical waveguide of the optical modulator PC is covered and patternends of the second resist mask RM2 are certainly located on theinsulating films IF1. Further, the second resist mask RM2 is formed inconsideration of the alignment error in the region D so that thesemiconductor layer SL to be the optical waveguide of the gratingcoupler GC is covered and pattern ends of the second resist mask RM2 arelocated on the semiconductor layer SL having the thickness in theformation of both ends of the grating coupler GC in the opticalwaveguide direction.

Next, as shown in FIG. 10, the semiconductor layer SL is processed byfull etching with using the second resist mask RM2 and the insulatingfilm IF1 as an etching mask. Thus, the semiconductor layer SL whichconstitutes the optical waveguide of the first optical signal line OT1and whose cross section orthogonal to the optical waveguide directionhas a quadrangular shape is formed in the region A, the semiconductorlayer SL which constitutes the optical waveguide of the second opticalsignal line OT2 and whose cross section orthogonal to the opticalwaveguide direction has a convex shape is formed in the region B, andthe semiconductor layer SL which constitutes the optical waveguide ofthe optical modulator PC and whose cross section orthogonal to theoptical waveguide direction has a convex shape is formed in the regionC. Further, the semiconductor layer SL which constitutes the opticalwaveguide of the grating coupler GC and whose cross section in theoptical waveguide direction has a convex shape is formed in the regionD.

Here, in the optical waveguide of the second optical signal line OT2formed in the region B and the optical waveguide of the opticalmodulator PC formed in the region C, the width of the projecting portionand the width of the slab portion in the direction orthogonal to theoptical waveguide direction are already determined by the insulatingfilm IF1. Therefore, the symmetry of the optical waveguide can beensured even when the alignment error of the second resist mask RM2occurs.

Hence, according to the first embodiment, it is not necessary toconsider the alignment error when forming the optical waveguide with arib structure, and the dimensional error in the half etching (forexample, about 1 nm) and the dimensional error in the full etching (for(for example, about 1 nm) mainly affect the symmetry of the opticalwaveguide with the rib structure. Since the alignment error is about 10nm in general, the symmetry of the optical waveguide with a ribstructure can be significantly improved.

In addition, since the trenches formed by the half etching are filledwith the insulating film IF1 and the upper surfaces of the semiconductorlayer SL and the insulating film IF1 are flat, it is possible to easilyform the second resist mask RM2 with the desired thickness anddimensions. Thus, it is possible to form the optical waveguide with arib structure with good reproducibility.

Next, as shown in FIG. 11, in the phase modulating part PM of theoptical modulator PC, a p type impurity is introduced by, for example,the ion implantation to the semiconductor layer SL constituting the slabportion on one side of the projecting portion, thereby forming the ptype semiconductor PR. Similarly, an n type impurity is introduced by,for example, the ion implantation to the semiconductor layer SLconstituting the slab portion on the other side of the projectingportion, thereby forming the n type semiconductor NR. The semiconductorlayer SL formed of a projecting portion between the p type semiconductorPR and the n type semiconductor NR serves as the core layer OW made ofintrinsic semiconductor.

Next, the first interlayer insulating film ID1 is formed so as to coverthe first optical signal line OT1, the second optical signal line OT2,the optical modulator PC and the grating coupler GC. The firstinterlayer insulating film ID1 is made of silicon oxide (SiO₂) formedby, for example, the plasma CVD (Chemical Vapor Deposition), and thethickness thereof is, for example, about 2 to 3 μm. Subsequently, theupper surface of the first interlayer insulating film ID1 is planarizedby, for example, the CMP.

Next, as shown in FIG. 12, after the first connection holes CT1 whichreach the p type semiconductor PR and the n type semiconductor NR of theoptical modulator PC are formed in the first interlayer insulating filmID1, a conductive film is formed to fill the first connection holes CT1,thereby forming the first plugs PL1 made of the conductive film embeddedtherein. The first plug PL1 is made of, for example, aluminum (Al) ortungsten (W).

Next, after a metal film, for example, an aluminum (Al) film isdeposited on the first interlayer insulating film ID1 by the sputteringor the like, the metal film is processed by dry etching using a resistmask, thereby forming the first layer wiring M1.

Next, the second interlayer insulating film ID2 is formed so as to coverthe first layer wiring M1 in the same manner as the first interlayerinsulating film ID1. The second interlayer insulating film ID2 is madeof silicon oxide (SiO₂) formed by, for example, plasma CVD, and thethickness thereof is, for example, 1 μm or more. Subsequently, the uppersurface of the second interlayer insulating film ID2 is planarized by,for example, the CMP.

Next, after the connection holes CT2 which reach the first layer wiringsM1 are formed in the second interlayer insulating film ID2, a conductivefilm is formed to fill the second connection holes CT2, thereby formingthe second plugs PL2 made of the conductive film embedded therein. Thesecond plug PL2 is made of, for example, aluminum (Al) or tungsten (W).

Next, after a metal film, for example, an aluminum (Al) film isdeposited on the second interlayer insulating film ID2 by the sputteringor the like, the metal film is processed by dry etching using a resistmask, thereby forming the second layer wiring M2.

Thereafter, the protection film. TC is formed so as to cover the secondlayer wiring M2. The protection film TC is made of, for example,example, silicon oxynitride (SiCN). Then, the protection film TC isprocessed to expose the upper surface of the second layer wiring M2.Herewith, the semiconductor device according to the first embodiment isalmost completed.

As described above, according to the first embodiment, the siliconoptical waveguide with a highly symmetric rib structure can be formed,and thus the optical device with low propagation loss can be obtained.

Second Embodiment Structure of Optical Device

An optical device according to the second embodiment will be describedwith reference to FIG. 13. FIG. 13 is a cross-sectional view showing amain part of the optical device according to the second embodiment, inwhich main parts of a first optical signal line constituted of anoptical waveguide whose cross section orthogonal to an optical waveguidedirection has a quadrangular shape, a second optical signal lineconstituted of an optical waveguide whose cross section orthogonal tothe optical waveguide direction has a convex shape, a phase modulatingpart of an optical modulator, a grating coupler and a germanium opticalreceiver are illustrated.

The difference from the first embodiment described above is that aninsulating film. IF2 is formed on each of the upper surfaces of theoptical waveguide constituting the first optical signal line OT1 and theprojecting portions of the optical waveguides with a rib structureconstituting the second optical signal line OT2, the optical modulatorPC and the grating coupler GC. The insulating film IF2 is made of, forexample, silicon nitride (Si₃N₄) or silicon oxide (SiO₂). The formationof the insulating film IF2 can prevent the surface roughness of thesemiconductor layer SL caused in a manufacturing method of an opticaldevice described later.

<<Manufacturing Method of Optical Device>>

A manufacturing method of an optical device according to the secondembodiment will be described in order of process with reference to FIG.14 to FIG. 19. FIG. 14 to FIG. 19 are cross-sectional views each showinga main part of an optical device in the manufacturing process accordingto the second embodiment. A region A indicates the first optical signalline (optical waveguide whose cross section orthogonal to the opticalwaveguide direction has a quadrangular shape), a region B indicates thesecond optical signal line (optical waveguide whose cross sectionorthogonal to the optical waveguide direction has a convex shape), aregion C indicates the phase modulating part of the optical modulatorand a region D indicates the grating coupler. The cross sectionorthogonal to the optical waveguide direction is shown for each of theregion A, the region B and the region C, and the cross section in theoptical waveguide direction is shown for the region D.

First, as shown in FIG. 14, the SOI substrate similar to that of thefirst embodiment described above is prepared. Next, the insulating filmIF2 is formed on the semiconductor layer SL. The insulating film IF2 ismade of, for example, silicon nitride (Si₃N₄) or silicon oxide (SiO₂),and the thickness thereof is, for example, about 2 to 10 nm.

Next, the first resist mask RM1 for processing the insulating film IF2is formed. The first resist mask RM1 is formed by, for example, applyingphotoresist on the insulating film IF2 and then patterning thephotoresist by performing the immersion exposure using the ArF excimerlaser (wavelength: 193 nm) and the development thereto.

Next, as shown in FIG. 15, the insulating film IF2 is etched with usingthe first resist mask RM1 as an etching mask. Thus, a hard mask HM madeof the insulating film IF2 is formed so as to expose the semiconductorlayer SL to be the slab portion of the optical waveguide of the secondoptical signal line OT2 in the region B, expose the semiconductor layerSL to be the slab portion of the optical waveguide of the opticalmodulator PC in the region C and expose the semiconductor layer SL to bethe slab portion of the optical waveguide of the grating coupler GC inthe region D. Namely, the hard mask HM, which determines the width ofthe projecting portion and the width of the slab portion in thedirection orthogonal to the optical waveguide direction in the region Band the region C and determines the width of the projecting portion andthe interval between the adjacent projecting portions (width of the slabportion) in the optical waveguide direction in the region D, is formed.Thereafter, the first resist mask RM1 is removed by oxygen (O₂) plasmaashing and then the RCA clean is performed.

Next, the semiconductor layer SL is processed by half etching with usingthe hard mask HM as an etching mask. Thus, a plurality of trenches areformed in the semiconductor layer SL. The depth of the trench is, forexample, about 70 nm. The trench determines the width of the projectingportion and the width of the slab portion in the direction orthogonal tothe optical waveguide direction in the region B and the region C anddetermines the width of the projecting portion and the interval betweenthe adjacent projecting portions (width of the slab portion) in theoptical waveguide direction in the region D. In addition, the thicknessof the semiconductor layer SL left under the trench corresponds to thethickness of the slab portion of each optical waveguide.

Further, the semiconductor layer SL to be the projecting portion of theoptical waveguide of the second optical signal line OT2 is formed in theregion B, the semiconductor layer SL to be the projecting portion of theoptical waveguide of the optical modulator PC is formed in the region C,and the semiconductor layer SL to be the projecting portion of theoptical waveguide of the grating coupler GC is formed in the region D.

As the etching gas for the half etching, for example, chlorine-based(Cl₂-based) gas, hydrogen bromide-based (HBr-based) gas, carbontetrafluoride-based (CF₄-based) gas or sulfur hexafluoride-based(SF₆-based) gas is used.

Next, as shown in FIG. 16, the insulating film IF1 is formed on thesemiconductor layer SL by, for example, the SA-CVD so as to fill each ofthe plurality of trenches formed in the semiconductor layer SL. Theinsulating film IF1 determines the width of the projecting portion andthe width of the slab portion in the direction orthogonal to the opticalwaveguide direction in the region B and the region C and determines thewidth of the projecting portion and the interval between the adjacentprojecting portions (width of the slab portion) in the optical waveguidedirection in the region D.

The insulating film IF1 is made of, for example, silicon oxide (SiO₂)and the thickness thereof is, for example, about 140 nm. Note that theinsulating film IF1 is not limited to silicon oxide (SiO₂), and anyinsulating film can be used as long as it can have etching selectivitywith respect to silicon (Si) constituting the semiconductor layer SL. Asdescribed above, however, if the refractive index is taken intoconsideration, silicon oxide (SiO₂) is preferable.

Next, the upper surface of the insulating film IF1 is ground by, forexample, the CMP, thereby filing each of the plurality of trenchesformed in the semiconductor layer SL with the insulating film IF1.

Incidentally, since the surface roughness of the semiconductor layer SLcauses the propagation loss of the optical waveguide, it is desirablethat the surface of the semiconductor layer SL is not ground. Therefore,in the second embodiment, the insulating film IF2 formed on thesemiconductor layer SL is used as a stopper when the insulating film IFis ground. Herewith, the surface of the semiconductor layer SL is notground, and thus the propagation loss of the optical waveguide due tothe surface roughness of the semiconductor layer SL can be prevented.

Next, as shown in FIG. 17, a second resist mask RM2 for processing thesemiconductor layer SL is formed. The second resist mask RM2 is formedby, for example, applying photoresist on the semiconductor layer SL andthen patterning the photoresist by performing the immersion exposureusing the ArF excimer laser (wavelength: 193 nm) and the developmentthereto.

Here, the second resist mask RM2 is formed in the region A so as tocover the semiconductor layer SL to be the optical waveguide of thefirst optical signal line OT1. Also, the second resist mask RM2 isformed in consideration of the alignment error in the region B so thatthe semiconductor layer SL to be the optical waveguide of the secondoptical signal line OT2 is covered and pattern ends of the second resistmask RM2 are certainly located on the insulating films IF1. In addition,the second resist mask RM2 is formed in consideration of the alignmenterror in the region C so that the semiconductor layer SL to be theoptical waveguide of the optical modulator PC is covered and patternends of the second resist mask RM2 are certainly located on theinsulating films IF1. Further, the second resist mask RM2 is formed inconsideration of the alignment error in the region D so that thesemiconductor layer SL to be the optical waveguide of the gratingcoupler GC is covered and pattern ends of the second resist mask RM2 arelocated on the semiconductor layer SL having the thickness in theformation of both ends of the grating coupler GC in the opticalwaveguide direction.

Next, as shown in FIG. 18, the insulating film IF2 is etched and thesemiconductor layer SL is processed by full etching with using thesecond resist mask RM2 and the insulating film IF1 as an etching mask.Thus, the semiconductor layer SL which constitutes the optical waveguideof the first optical signal line OT1 and whose cross section orthogonalto the optical waveguide direction has a quadrangular shape is formed inthe region A, the semiconductor layer SL which constitutes the opticalwaveguide of the second optical signal line OT2 and whose cross sectionorthogonal to the optical waveguide direction has a convex shape isformed in the region B, and the semiconductor layer SL which constitutesthe optical waveguide of the optical modulator PC and whose crosssection orthogonal to the optical waveguide direction has a convex shapeis formed in the region C. Further, the semiconductor layer SL whichconstitutes the optical waveguide of the grating coupler GC and whosecross section in the optical waveguide direction has a convex shape isformed in the region D.

Here, in the optical waveguide of the second optical signal line OT2formed in the region B and the optical waveguide of the opticalmodulator PC formed in the region C, the width of the projecting portionand the width of the slab portion in the direction orthogonal to theoptical waveguide direction are already determined by the insulatingfilm IF1. Therefore, the symmetry of the optical waveguide can beensured even when the alignment error of the second resist mask RM2occurs.

Hence, according to the second embodiment, it is not necessary toconsider the alignment error when forming the optical waveguide with arib structure, and the dimensional error in the half etching (forexample, about 1 nm) and the dimensional error in the full etching (for(for example, about 1 nm) mainly affect the symmetry of the opticalwaveguide with the rib structure. Since the alignment error is about 10nm in general, the symmetry of the optical waveguide with a ribstructure can be significantly improved.

In addition, since the trenches formed by the half etching are filledwith the insulating film IF1 and the upper surfaces of the insulatingfilms IF1 and IF2 are flat, it is possible to easily form the secondresist mask RM2 with the desired thickness and dimensions. Thus, it ispossible to form the optical waveguide with a convex shape with goodreproducibility.

Next, in the same manner as that of the first embodiment describedabove, the first layer wiring M1, the second layer wiring M2 and othersare formed and then the protection film TC covering the second layerwiring M2 is formed as shown in FIG. 19. Herewith, the semiconductordevice according to the second embodiment is almost completed.

As described above, according to the second embodiment, the surfaceroughness of the semiconductor layer SL constituting the opticalwaveguide can be reduced, and thus the optical device with lowerpropagation loss than the first embodiment described above can beobtained.

Third Embodiment Structure of Optical Device

An optical device according to the third embodiment will be describedwith reference to FIG. 20. FIG. 20 is a cross-sectional view showing amain part of the optical device according to the third embodiment, inwhich main parts of a first optical signal line constituted of anoptical waveguide whose cross section orthogonal to an optical waveguidedirection has a quadrangular shape, a second optical signal lineconstituted of an optical waveguide whose cross section orthogonal tothe optical waveguide direction has a convex shape, a phase modulatingpart of an optical modulator, a grating coupler and a germanium opticalreceiver are illustrated.

The difference from the first embodiment described above is that theinsulating film IF1 is formed also on each of the upper surfaces of theoptical waveguide constituting the first optical signal line OT1 and theprojecting portions of the optical waveguides with a rib structureconstituting the second optical signal line OT2, the optical modulatorPC and the grating coupler GC. By forming the insulating film IF1 oneach of the upper surfaces of the optical waveguide constituting thefirst optical signal line OT1 and the projecting portions of the opticalwaveguides with a rib structure constituting the second optical signalline OT2, the optical modulator PC and the grating coupler GC, it ispossible to prevent the surface roughness of the semiconductor layer SLcaused in a manufacturing method of an optical device described later.

<<Manufacturing Method of Optical Device>>

A manufacturing method of an optical device according to the thirdembodiment will be described in order of process with reference to FIG.21 to FIG. 26. FIG. 21 to FIG. 26 are cross-sectional views each showinga main part of an optical device in the manufacturing process accordingto the third embodiment. A region A indicates the first optical signalline (optical waveguide whose cross section orthogonal to the opticalwaveguide direction has a quadrangular shape), a region B indicates thesecond optical signal line (optical waveguide whose cross sectionorthogonal to the optical waveguide direction has a convex shape), aregion C indicates the phase modulating part of the optical modulatorand a region D indicates the grating coupler. The cross sectionorthogonal to the optical waveguide direction is shown for each of theregion A, the region B and the region C, and the cross section in theoptical waveguide direction is shown for the region D.

First, as shown in FIG. 21, the SOI substrate similar to that of thefirst embodiment described above is prepared and the first resist maskRM1 for processing the semiconductor layer SL is formed. The firstresist mask RM1 is formed by, for example, applying photoresist on thesemiconductor layer SL and then patterning the photoresist by performingthe immersion exposure using the ArF excimer laser (wavelength: 193 nm)and the development thereto.

In this case, the first resist mask RM1 is formed so as to expose thesemiconductor layer SL to be the slab portion of the optical waveguideof the second optical signal line OT2 in the region B, expose thesemiconductor layer SL to be the slab portion of the optical waveguideof the optical modulator PC in the region C and expose the semiconductorlayer SL to be the slab portion of the optical waveguide of the gratingcoupler GC in the region D. Namely, the first resist mask RM1, whichdetermines the width of the projecting portion and the width of the slabportion in the direction orthogonal to the optical waveguide directionin the region B and the region C and determines the width of theprojecting portion and the interval between the adjacent projectingportions (width of the slab portion) in the optical waveguide directionin the region D, is formed.

Next, as shown in FIG. 22, the semiconductor layer SL is processed byhalf etching with using the first resist mask RM1 as an etching mask.Thus, a plurality of trenches are formed in the semiconductor layer SL.The depth of the trench is, for example, about 70 nm. The trenchdetermines the width of the projecting portion and the width of the slabportion in the direction orthogonal to the optical waveguide directionin the region B and the region C and determines the width of theprojecting portion and the interval between the adjacent projectingportions (width of the slab portion) in the optical waveguide directionin the region D. In addition, the thickness of the semiconductor layerSL left under the trench corresponds to the thickness of the slabportion of each optical waveguide.

Further, the semiconductor layer SL to be the projecting portion of theoptical waveguide of the second optical signal line OT2 is formed in theregion B, the semiconductor layer SL to be the projecting portion of theoptical waveguide of the optical modulator PC is formed in the region C,and the semiconductor layer SL to be the projecting portion of theoptical waveguide of the grating coupler GC is formed in the region D.

As the etching gas for the half etching, for example, chlorine-based(Cl₂-based) gas, hydrogen bromide-based (HBr-based) gas, carbontetrafluoride-based (CF₄-based) gas or sulfur hexafluoride-based(SF₆-based) gas is used. After the half etching, the first resist maskRM1 is removed by oxygen (O₂) plasma ashing and then the RCA clean isperformed. Thereafter, natural oxide film and others formed on thesurface of the semiconductor layer SL are removed by the wet etching.

Next, as shown in FIG. 23, the insulating film IF1 is formed on thesemiconductor layer SL by, for example, the SA-CVD so as to fill each ofthe plurality of trenches formed in the semiconductor layer SL. Theinsulating film IF1 determines the width of the projecting portion andthe width of the slab portion in the direction orthogonal to the opticalwaveguide direction in the region B and the region C and determines thewidth of the projecting portion and the interval between the adjacentprojecting portions (width of the slab portion) in the optical waveguidedirection in the region D.

The insulating film IF1 is made of, for example, silicon oxide (SiO₂)and the thickness thereof is, for example, about 140 nm. Note that thatthe insulating film IF1 is not limited to silicon oxide (SiO₂), and anyinsulating film can be used as long as it can have etching selectivitywith respect to silicon (Si) constituting the semiconductor layer SL. Asdescribed above, however, if the refractive index is taken intoconsideration, silicon oxide (SiO₂) is preferable.

Next, the upper surface of the insulating film IF1 is ground by, forexample, the CMP, thereby filing each of the plurality of trenchesformed in the semiconductor layer SL with the insulating film IF1.

Incidentally, since the surface roughness of the semiconductor layer SLcauses the propagation loss of the optical waveguide as described in thesecond embodiment above, it is desirable that the surface of thesemiconductor layer SL is not ground. Therefore, in the thirdembodiment, when grinding the insulating film IF1, the insulating filmIF1 is left on the upper surface of the semiconductor substrate SLwithout grinding it until the upper surface of the semiconductor layerSL is exposed. Consequently, the surface of the semiconductor layer SLis not ground, and thus the propagation loss of the optical waveguidedue to the surface roughness of the semiconductor layer SL can beprevented. The thickness of the insulating film IF1 left on the uppersurface of the semiconductor layer SL having no trench formed therein(hereinafter, referred to as protection insulating film IF1 a) is, forexample, about 2 to 10 nm.

Next, as shown in FIG. 24, a second resist mask RM2 for processing thesemiconductor layer SL is formed. The second resist mask RM2 is formedby, for example, applying photoresist on the semiconductor layer SL andthen patterning the photoresist by performing the immersion exposureusing the ArF excimer laser (wavelength: 193 nm) and the developmentthereto.

Here, the second resist mask RM2 is formed in the region A so as tocover the semiconductor layer SL to be the optical waveguide of thefirst optical signal line OT1. Also, the second resist mask RM2 isformed in consideration of the alignment error in the region B so thatthe semiconductor layer SL to be the optical waveguide of the secondoptical signal line OT2 is covered and pattern ends of the second resistmask RM2 are certainly located on the insulating films IF1. In addition,the second resist mask RM2 is formed in consideration of the alignmenterror in the region C so that the semiconductor layer SL to be theoptical waveguide of the optical modulator PC is covered and patternends of the second resist mask RM2 are certainly located on theinsulating films IF1. Further, the second resist mask RM2 is formed inconsideration of the alignment error in the region D so that thesemiconductor layer SL to be the optical waveguide of the gratingcoupler GC is covered and pattern ends of the second resist mask RM2 arelocated on the semiconductor layer SL having the thickness in theformation of both ends of the grating coupler GC in the opticalwaveguide direction.

Next, as shown in FIG. 25, the protection insulating film IF1 a isetched with using the second resist mask RM2 as an etching mask, and thesemiconductor layer SL is processed by full etching with using thesecond resist mask RM2 and the insulating film IF1 as an etching mask.Thus, the semiconductor layer SL which constitutes the optical waveguideof the first optical signal line OT1 and whose cross section orthogonalto the optical waveguide direction has a quadrangular shape is formed inthe region A, the semiconductor layer SL which constitutes the opticalwaveguide of the second optical signal line OT2 and whose cross sectionorthogonal to the optical waveguide direction has a convex shape isformed in the region B, and the semiconductor layer SL which constitutesthe optical waveguide of the optical modulator PC and whose crosssection orthogonal to the optical waveguide direction has a convex shapeis formed in the region C. Further, the semiconductor layer SL whichconstitutes the optical waveguide of the grating coupler GC and whosecross section in the optical waveguide direction has a convex shape isformed in the region D.

Here, in the optical waveguide of the second optical signal line OT2formed in the region B and the optical waveguide of the opticalmodulator PC formed in the region C, the width of the projecting portionand the width of the slab portion in the direction orthogonal to theoptical waveguide direction are already determined by the insulatingfilm IF1. Therefore, the symmetry of the optical waveguide can beensured even when the alignment error of the second resist mask RM2occurs.

Hence, according to the third embodiment, it is not necessary toconsider the alignment error when forming the optical waveguide with arib structure, and the dimensional error in the half etching (forexample, about 1 nm) and the dimensional error in the full etching (forexample, about 1 nm) mainly affect the symmetry of the optical waveguidewith the rib structure. Since the alignment error is about 10 nm ingeneral, the symmetry of the optical waveguide with a rib structure canbe significantly improved.

In addition, since the trenches formed by the half etching are filledwith the insulating film IF1 and the upper surfaces of the insulatingfilm IF1 and the protection insulting film IF1 a are flat, it ispossible to easily form the second resist mask RM2 with the desiredthickness and dimensions. Thus, it is possible to form the opticalwaveguide with a convex shape with good reproducibility.

Next, in the same manner as that of the first embodiment describedabove, the first layer wiring M1, the second layer wiring M2 and othersare formed and then the protection film TC covering the second layerwiring M2 is formed as shown in FIG. 26. Herewith, the semiconductordevice according to the third embodiment is almost completed.

As described above, according to the third embodiment, the surfaceroughness of the semiconductor layer SL constituting the opticalwaveguide can be reduced, and thus the optical device with lowerpropagation loss than the first embodiment described above can beobtained.

In the foregoing, the invention made by the inventors of the presentinvention has been concretely described based on the embodiments.However, it is needless to say that the present invention is not limitedto the foregoing embodiments and various modifications and alterationscan be made within the scope of the present invention.

What is claimed is:
 1. A semiconductor device comprising: a firstinsulating film formed on a main surface of a semiconductor substrate;an optical waveguide which is formed on an upper surface of the firstinsulating film and includes a first plate portion made of asemiconductor layer and having a first thickness and second plateportions made of the semiconductor layer disposed on both sides of thefirst plate portion and having a second thickness smaller than the firstthickness; a second insulating film formed on each of upper surfaces ofthe second plate portions and on an outer side of side surfaces of aprotruding portion of the first plate; and a third insulating filmformed on the upper surface of the first insulating film so as to coverthe optical waveguide and the second insulating film.
 2. Thesemiconductor device according to claim 1, wherein the first plateportion and the second plate portions are integrally formed.
 3. Thesemiconductor device according to claim 1, wherein a height from theupper surface of the first insulating film to an upper surface of thefirst plate portion is the same as a height from the upper surface ofthe first insulating film to an upper surface of the second insulatingfilm.
 4. The semiconductor device according to claim 1, wherein thefirst insulating film, the second insulating film and the thirdinsulating film are made of silicon oxide and the first plate portionand the second plate portion are made of silicon.
 5. The semiconductordevice according to claim 1, wherein the first plate extends in anoptical waveguide direction.
 6. The semiconductor device according toclaim 1, wherein a fourth insulating film is formed on an upper surfaceof the first plate portion and a height from the upper surface of thefirst insulating film to an upper surface of the fourth insulating filmis the same as a height from the upper surface of the first insulatingfilm to an upper surface of the second insulating film.
 7. Thesemiconductor device according to claim 6, wherein the fourth insulatingfilm is integrally formed with the second insulating film.
 8. Amanufacturing method of a semiconductor device having an opticalwaveguide constituted of a first plate portion made of a semiconductorlayer extending in an optical waveguide direction and having a firstthickness and second plate portions made of the semiconductor layerdisposed on both sides of the first plate portion in a width directionthereof and having a second thickness smaller than the first thickness,the method comprising the steps of: (a) preparing a substrateconstituted of a semiconductor substrate, a first insulating film on anupper surface of the semiconductor substrate and a semiconductor layerwith the first thickness on an upper surface of the first insulatingfilm; (b) forming a first resist mask having openings at positions ofsecond-plate-portion regions in which the second plate portions are tobe formed, on an upper surface of the semiconductor layer; (c)performing first etching to the semiconductor layer with using the firstresist mask as an etching mask, thereby forming trenches in thesecond-plate-portion regions while leaving the semiconductor layer withthe second thickness; (d) forming a second insulating film on the uppersurface of the semiconductor layer including insides of the trenches andthen grinding the second insulating film to fill the trenches with thesecond insulating film; (e) forming a second resist mask which coversthe semiconductor layer of a first-plate-portion region in which thefirst plate portion is to be formed and whose pattern ends are locatedon upper surfaces of the second insulating films, on the upper surfaceof the semiconductor layer and on the upper surfaces of the secondinsulating films; and (f) performing second etching to the semiconductorlayer with using the second resist mask and the second insulating filmsas an etching mask, thereby forming the first plate portion with thefirst thickness in the first-plate-portion region and the second plateportions with the second thickness in the second-plate-portion regions.9. The manufacturing method of a semiconductor device according to claim8, wherein, in the step (d), a height from the upper surface of thefirst insulating film to an upper surface of the first plate portion ismade the same as a height from the upper surface of the first insulatingfilm to the upper surface of the second insulating film.
 10. Themanufacturing method of a semiconductor device according to claim 8,wherein, in the step (d), the second insulating film with a thickness of2 to 10 nm is left on the upper surface of the semiconductor layer inthe first-plate-portion region.
 11. The manufacturing method of asemiconductor device according to claim 8, wherein the first insulatingfilm and the second insulating film are made of silicon oxide and thesemiconductor layer is made of silicon.
 12. A manufacturing method of asemiconductor device having an optical waveguide constituted of a firstplate portion made of a semiconductor layer extending in an opticalwaveguide direction and having a first thickness and second plateportions made of the semiconductor layer disposed on both sides of thefirst plate portion in a width direction thereof and having a secondthickness smaller than the first thickness, the method comprising thesteps of: (a) preparing a substrate constituted of a semiconductorsubstrate, a first insulating film on a main surface of thesemiconductor substrate and a semiconductor layer with the firstthickness on an upper surface of the first insulating film; (b) forminga second insulating film on an upper surface of the semiconductor layer;(c) forming a first resist mask having openings at positions ofsecond-plate-portion regions in which the second plate portions are tobe formed, on an upper surface of the second insulating film; (d)performing first etching to the second insulating film with using thefirst resist mask as an etching mask, thereby forming a hard mask madeof the second insulating film having openings at positions of thesecond-plate-portion regions; (e) performing second etching to thesemiconductor layer with using the hard mask as an etching mask, therebyforming trenches in the second-plate-portion regions while leaving thesemiconductor layer with the second thickness; (f) forming a thirdinsulating film on the upper surface of the second insulating filmincluding insides of the trenches and then grinding the third insulatingfilm to fill the trenches with the third insulating film; (g) forming asecond resist mask which covers the second insulating film in afirst-plate-portion region in which the first plate portion is to beformed and whose pattern ends are located on the third insulating films,on the upper surface of the second insulating film and on an uppersurface of the third insulating film; and (h) performing third etchingto the second insulating film and the semiconductor layer with using thesecond resist mask and the third insulating film as an etching mask,thereby forming the first plate portion having the first thickness inthe first-plate-portion region and forming the second plate portionshaving the second thickness in the second-plate-portion regions.
 13. Themanufacturing method of a semiconductor device according to claim 12,wherein, in the step (f), a height from the upper surface of the firstinsulating film to the upper surface of the second insulating film ismade the same as a height from the upper surface of the first insulatingfilm to the upper surface of the third insulating film.
 14. Themanufacturing method of a semiconductor device according to claim 12,wherein a thickness of the second insulating film is 2 to 10 nm.
 15. Themanufacturing method of a semiconductor device according to claim 12,wherein the first insulating film and the third insulating film are madeof silicon oxide, the second insulating film is made of silicon nitrideor silicon oxide and the semiconductor layer is made of silicon.